The present invention relates to a magnetic carrier for electrophotography, and more particularly, to a magnetic carrier for electrophotography composed of spherical composite particles which have a small bulk density, an excellent fluidity, an appropriate saturation magnetization, especially a saturation magnetization of about 20 to 90 emu/g, an appropriate specific gravity (i.e., true specific gravity), especially a specific gravity of about 2.5 to 5.2, and a comparatively high electric resistance, especially an electric resistance of about 10.sup.10 to 10.sup.14 .OMEGA.cm.
As is known, in the method adopted by electrophotography, a photoconductive material such as selenium, OPC (organic semiconductor) and a-Si is used for a photoreceptor, an electrostatic latent image is formed by various means, and toner electrified to the opposite polarity to the polarity of the latent image is adhered to the latent image by an electrostatic force by magnetic brush development or the like so as to develop the image.
In the developing process, particles so called a carrier is used. The carrier provides a toner with an appropriate amount of positive or negative electric charge by frictional electrification and carries the toner, by utilizing a magnetic force, to a developing area in the vicinity of the surface of a photoreceptor with the latent image formed thereon, through a developing sleeve which accommodates a magnet.
With the increasingly wide use of electrophotography in copying machines, printers, etc., electrophotography has recently been required to deal with various objects such as fine lines, small letters, photograph and colored manuscripts. Electrophotography is also required to improve the picture quality, to enhance the dignity, to increase the speed of the copying and to enable continuous processing of the copying. There requests are expected to be increasing more and more.
As a carrier, iron powder carrier, ferrite carrier, binder-type carrier (composite particles of fine magnetic particles dispersed in a resin), etc. have conventionally been developed and put to practical use.
An iron powder carrier which has a shape of flakes, sponges or spheres have a specific gravity of about 7 to 8 and a bulk density as large as 3 to 4 g/cm.sup.3, so that it requires a large driving force when stirred in a developing machine, which is apt to lead to much mechanical wear, exhaustion of the toner, deterioration in the electrification property of the carrier itself, and a damage in the photoreceptor.
A ferrite carrier is composed of spherical particles, and has a specific gravity of about 4.5 to 5.5 and a bulk density of about 2 to 3 g/cm.sup.3, so that it can solve the problem of a heavy weight which is suffered from by an iron powder carrier, to a certain degree but it is still insufficient.
A binder-type carrier has a bulk density as small as not more than 2.5 g/cm.sup.2, and since it is comparatively easy to form spherical particles therefrom which has little distortion in shape and a high particle strength, it has an excellent fluidability. In addition, it is possible to control the particle size of the binder-type carrier in a wide range. The binder-type carrier is thus expected most as a carrier for a developing sleeve, a high-speed copying machine in which the number of revolutions of the magnet in a developing sleeve is large, a high-speed laser beam printer of a general-purpose computer, etc.
The known resins used for a binder-type carrier are roughly divided into thermoplastic resins such as vinyl-based resins, styrene-based resins and acrylic-based resins, and thermosetting resins such as phenol-based resins, melamine-based resins and epoxy-based resins. Thermoplastic resins which are easy to granulate are generally used and thermosetting resins are considered to have a problem in practical use because it is difficult to form spherical particles therefrom.
On the other hand, since thermosetting resins are superior in the durability, the shock resistance and the heat resistance to thermoplastic resins, a binder-type carrier (composite particles) composed of inorganic particles and a thermosetting resin having these merits is strongly demanded, and composite particles using a phenol resin as a thermosetting resin and ferromagnetic particles as inorganic particles is known (Japanese Patent Application Laid-Open (KOKAI) Nos. 2-220068/1990 and 4-100850/1992). However, there is no end to the demand for higher capacity of a binder-type carrier and it is required to have appropriately controlled magnetization value, specific gravity and electric resistance in addition to the above-described properties.
A carrier is firstly required to have an appropriate saturation magnetization, especially a saturation magnetization of about 20 to 90 emu/g. In other words, when the saturation magnetization is in the range of 20 to 90 emu/g, it is possible to obtain a good image. If the saturation magnetization is not less than 20 emu/g, there is little possibility of exhibiting a carrier adherence phenomenon which is a phenomenon of a carrier forming what is called an "ear" of a magnet brush on a sleeve leaving from the ear and flying and adhering to the photoreceptor due to a lower magnetic force. If the saturation magnetization is not more than 90 emu/g, it is possible to lower the mechanical strength applied to a magnetic toner, thereby preventing the magnetic toner from crushing. A carrier is therefore required to have a saturation magnetization in the range of 20 to 90 emu/g.
A carrier is secondly required to electrify a toner quickly. In other words, it is important that a carrier is mixed well with a toner. For this purpose, a carrier is required to have an appropriate specific gravity, especially, a specific gravity of about 2.5 to 5.2. If a carrier has a large specific gravity, it is mixed well with a toner. But in order to prevent a carrier from doing damage to the toner, for example, to prevent exhaustion of the toner, and to reduce the size and the weight of a developing machine, a carrier having a small specific gravity is desirable. Therefore, a carrier is required to have a specific gravity of about 2.5 to 5.2.
A carrier is thirdly required to have a comparatively high electric resistance, especially an electric resistance of about 10.sup.10 to 10.sup.14 .OMEGA.cm. If a carrier has a volume intrinsic resistance as low as not more than 10.sup.6 .OMEGA.cm, the carrier adheres to the image portion of the photoreceptor by injection of charge from the sleeve, or the charge releases from the latent image, which leads to a disturbance in the latent image or a defect of the image.
In order to solve this problem, a method of covering the surfaces of carrier particles with a resin so as to increase the electric resistance of the carrier is proposed (Japanese Patent Application Laid-Open (KOKAI) Nos. 47-13954/1972 and 57-660/1982).
However, since such a resin is an insulator, the electric resistance of the carrier itself becomes much higher than 10.sup.14 .OMEGA.cm, and the carrier charge is unlikely to leak. In addition, the charge of the toner is increased and as a result, the image produced has an edge effect but the density in the center portion becomes very low in an image having a large area. Consequently, a carrier is required to have a comparatively high electric resistance, particularly a volume intrinsic resistance of about 10.sup.10 to 10.sup.14 .OMEGA.cm.
Some attempts have conventionally been made to produce a binder-type carrier having an appropriate electric resistance. For example, a magnetic powder dispersion-type carrier with a fine inorganic oxide powder adhered to the surfaces of at least a part thereof by adding the fine inorganic oxide powder to the carrier in advance (Japanese Patent Application Laid-Open (KOKAI) No. 4-124677/1992), and magnetic particles dispersion-type carrier with fine conductive particles having a volume resistance of not more than 10.sup.12 .OMEGA.cm added to the surfaces thereof (Japanese Patent Application Laid-Open (KOKAI) No. 5-273789/1993) are proposed.
A magnetic carrier composed of spherical composite particles which have a small bulk density and an excellent fluidity, and which have all of the following properties with a good balance: an appropriate saturation magnetization, especially a saturation magnetization of about 20 to 90 emu/g, an appropriate specific gravity, especially a specific gravity of about 2.5 to 5.2, and a comparatively high electric resistance, especially an electric resistance of about 10.sup.10 to 10.sup.14 .OMEGA.cm, is now in the strongest demand, but such a magnetic carrier has never been provided.
The binder-type carriers composed of spherical phenol resin composite particles containing ferromagnetic particles described in Japanese Patent Application Laid-Open (KOKAI) Nos. 2-220068/1990 and 4-100850/1992 are not aimed at the control of the electric resistance due to the ratio of the particle diameters of the ferromagnetic particles and the non-magnetic particles. The electric resistances of these carriers are less than 10.sup.10 .OMEGA.cm, which is beyond the range described above.
Neither the carrier described in Japanese Patent Application Laid-Open (KOKAI) No. 4-124677/1992 nor the carrier described in Japanese Patent Application Laid-Open KOKAI) 5-273789/1993 can be said to sufficiently meet the above-described demands.
Each of these carriers described in Japanese Patent Application Laid-Open (KOKAI) Nos. 4-124677/1992 and 5-273789/1993 is produced by adhering a fine inorganic oxide powder to the surfaces of the composite particles containing ferromagnetic particles, and since the carrier does not have a coating layer of the fine inorganic oxide powder uniformly dispersed in a resin matrix, the fine inorganic oxide powder is easily peeled off by a mechanical shock.
Accordingly, a magnetic carrier composed of spherical composite particles which has a small bulk density, an excellent fluidity, and which satisfies all of the conditions of an appropriate saturation magnetization, especially a saturation magnetization of about 20 to 90 emu/g, an appropriate specific gravity, especially a specific gravity of about 2.5 to 5.2, and a comparatively high electric resistance, especially an electric resistance of about 10.sup.10 to 10.sup.14 .OMEGA.cm is now demanded.
As a result of the studies undertaken by the present inventors so as to meet the above-mentioned demand, it has been found that by dispersing ferromagnetic iron compound particles and non-magnetic metal oxide particles which have a number-average particle diameter larger than that of the ferromagnetic iron compound particles to a phenol-based resin as a binder resin so that the total amount of the ferromagnetic iron compound particles and the non-magnetic metal oxide particles is 80 to 99 wt % in a magnetic carrier for electrophotography, the obtained spherical composite particles are useful as a magnetic carrier for electrophotography which is capable of realizing high picture quality, high dignity, high speed of the copying and continuous processing of the copying. The present invention has been achieved on the basis of this finding.